A particle with negative kinetic energy tends to the minimum of
the action and therefore tends to achieve a maximal speed. If such
a particle is free, it cannot accelerate, as this would violate
energy conservation. Hence, a free ghost does not produce any
harm to the environment, being isolated from it.
However, in the case when there is an interaction of a ghost with
healthy fields, the argument about energy conservation in a closed
system does not work. Since any physical system tends to the
state with minimal action, a ghost tends to accelerate, transmitting
the extra positive energy to the healthy fields interacting with it,
in the form of the quantum or classical emission of the
corresponding particles. A systematic study of this situation at the
quantum level has been given by Veltman in [1]}.
